1. Field of the Invention
The present invention relates to a plasma-processing method and an apparatus for carrying out the same and, more particularly, to a plasma-processing method suitable for plasma-etching a workpiece, such as a film coating a semiconductor substrate, and an apparatus for carrying out the same.
2. Description of the Related Art
Etching techniques that vary the frequency of a RF bias voltage (radio-frequency bias voltage) to be applied to a substrate, such as a semiconductor wafer, when etching a film formed over the surface of a substrate are disclosed, for instance, in Japanese Patent Laid-open (Kokai) Nos. 4-27119 and 4-14853. The technique disclosed in Japanese Patent Laid-open (Kokai) No. 4-27119 employs frequency control in combination with RF bias power control because it is difficult to control ion energy only by RF bias power control. The specification of this known invention describes that there is the tendency that ion energy decreases with the increase of the bias frequency, and vice versa. Although effective in controlling anisotropy and reducing damages in the substrate, this known technique takes nothing into consideration about etch selectivity, i.e., the ratio of an etching rate with a material to be etched to the etching rate with an under layer under a layer of the material to be etched, and does not show any method of setting a frequency to provide a high etch selectivity.
The technique disclosed in Japanese Patent Laid-open (Kokai) No. 4-14853 utilizing a phenomenon that an etching reaction is caused principally by radicals when the frequency of the RF bias is high or principally by ions when the frequency of the RF bias is low uses an RF bias having a high frequency for etching a metallic suicide layer having a high melting point and uses a RF bias having a low frequency for etching a polycrystalline silicon layer. It is described on p. 7, left col. of the specification of Japanese Patent Laid-open (Kokai) No. 4-14853, in connection with process conditions for etching a polycrystalline silicon layer, that a high etch selectivity was obtained in an embodiment when the frequency of the RF bias was 800 KHz. However, there is no discussion about a method of setting a frequency that provides a maximum etch selectivity under various process conditions and no reference is made to an etching method that realizes a high etch selectivity at a high etching rate.